1. Technical Field
This invention relates to a screw which may be used in plasticizing a fiber-reinforced thermoplastic resin, a plasticizing apparatus having such screw mounted thereto, a method for molding a fiber-reinforced thermoplastic resin which has been plasticized by using such plasticizing apparatus, and an article molded from a fiber-reinforced thermoplastic resin by such method.
2. Background of the Invention
Pellets of long fiber-reinforced thermoplastic resins are produced by impregnating a bundle of reinforcing continuous fibers such as glass fibers with a molten resin and cutting the resin-impregnated fibers, and such pellets have the characteristic feature that the pellet contains the reinforcing fiber of the length equal to the pellet.
As a consequence, length of the long fibers remaining in the article produced by injection molding such pellets of fiber-reinforced thermoplastic resin is longer than the length of the fiber remaining in the article molded from a short-fiber-reinforced thermoplastic resin. The article molded from the pellets of fiber-reinforced thermoplastic resin, therefore, exhibits excellent mechanical properties and durability comparable to those of continuous-fiber-reinforced composite, as well as a moldability comparable to that of a short-fiber-reinforced thermoplastic resin.
In the state of the art, plasticization of a long-fiber-reinforced thermoplastic resin is conducted in a plasticizing apparatus having a general-purpose plasticizing screw for an ordinary thermoplastic resin which is not specifically designed for the blending of the reinforcing fiber and under the molding conditions generally used for molding thermoplastic resins. Such plasticization in the conventional apparatus was associated with the problems such as decrease in the length of the reinforcing fiber by breakage during the plasticization, and clumps (which is the state of insufficient opening of the fiber bundle) remaining in the molded article when the shear force was reduced to suppress the reinforcing fiber breakage.
FIGS. 3 and 4 are side views of exemplary conventional plasticizing screws for a thermoplastic resin. In the figures, the numeral 1 represents the screw; Sf represents feed section where the resin is heated and conveyed; Sc represents compression section where the heated resin conveyed from the feed section Sf is melted and kneaded; Sm represents metering section where the molten kneaded resin conveyed from the compression section Sc is conveyed to nozzle 6; Lf represents screw lead length of the feed section Sf; Lc.sub.1, Lc.sub.2 and Lc.sub.i represent first, second, and the i-th screw lead lengths of the compression section Sc; Lm represents screw lead length of the metering section Sm; Hf represents screw channel depth of the feed section Sf; Hm represents screw channel depth of the metering section Sm; Wf represents width of the screw channel of the feed section Sf; Wc.sub.i represents width of the screw channel of the compression section Sc; Wm represents width of the screw channel of the metering section Sm; and D represents screw diameter.
The feed section Sf is the section where the material fed from the hopper 7 is conveyed in forward direction while the material is preliminarily heated before conveying into the subsequent compression section Sc. The compression section Sc is the section where the resin melts by the shearing action in combination with the externally applied heat and where resin pressure is generated to convey the thus molten resin to the subsequent metering section Sm. The metering section Sm is the section where the molten resin is further kneaded for complete uniformity.
The screw lead length is the distance of resin advancement in axial direction per 1 revolution of the screw when it is postulated that the screw channel is 100% filled with the resin.
The screw diameter is generally constant along the overall length of the screw from the feed section Sf to the metering section Sm. The relationship of the overall length L of the screw 1 to the screw diameter D, namely, L/D ratio, is generally in the range of 15 to 25. The feed section Sf, the compression section Sc and the metering section Sm are generally of the ratio of the length of screw sections in the range of from 2:1:1 to 3:2:1.
In the conventional screw 1, the feed section Sf has the screw channel depth Hf greater than the screw channel depth Hm of the metering section Sm, and the decrease in the channel cross-sectional area due to such decrease in the channel depth in the forward direction of the screw generates the shear force required for the melting and kneading of the resin.
The screw channel depth is in the range of 0.13 D to 0.18D in the feed section Sf, 0.03D to 0.08D in the metering section Sm, and the compression ratio represented by "screw channel depth Hf of the feed section Sf/screw channel depth Hm of the feed section Sm" is generally in the range of 1.8 to 3.5. When the screw has a larger compression ratio, shear force applied from somewhere around the compression section Sc will be stronger. The screw lead length is in the range of 0.9D to 1.1D, and is constant from the feed section Sf to the metering section Sm.
Next, an apparatus for plasticizing a thermoplastic resin having mounted thereto the plasticizing screw as described above is described.
Thermoplastic resins are generally plasticized in the plasticizing apparatus as shown in FIG. 4, and the plasticized resin is injected into the mold 9a, 9b as shown in FIG. 5 and is molded. In FIGS. 4 and 5, 1 is the screw, 2 is non-return check valve assembly, 3 is screw head, 4 is cylinder, 4a is cylinder head, 5 is heater, 6 is the nozzle, 7 is the hopper, 8 is screw reciprocating drive including the screw rotating device and the hydraulic device, 9a and 9b are the mold, and 10 is cavity.
It has been the findings of the inventors of the present invention that, when a long-fiber-reinforced thermoplastic resin having the fiber length of 9 mm is molded in a plasticizing apparatus having mounted thereto a conventional plasticizing screw generally used for a thermoplastic resin under normal molding conditions, the strong shear force applied by each parts of the screw results in decrease in the length of the fibers remaining in the molded article to the level of about 0.5 mm although clump formation may be suppressed. As a result, mechanical properties of the molded articles were not substantially improved over those of the articles molded from the short-fiber-reinforced thermoplastic resin (fiber length: about 0.3 mm).
In view of such situation, JP-A 2-292008 discloses a method wherein fiber breakage during the plasticization is prevented by designing the screw channels with the screw channel depth of over 5 mm along the full length of the screw. This method was to some extent effective in preventing the decrease in length of the fiber remaining in the molded article. The mechanical properties of the molded article, however, were not substantially improved.
The JP-A 2-292008 also describes the limitation of the ratio L/D of the overall length L to the diameter D of the screw to the range of 7 to 15. Such limitation may have the effect of reducing the period during which the shear force of the screw is applied to the long-fiber-reinforced thermoplastic resin. The overall shear force applied to the long-fiber-reinforced thermoplastic resin, on the other hand, is reduced by such limitation, and clumps are likely to remain in the molded article to invite stress concentration, and hence, reduced strength of the molded article.
An improvement for such susceptibility to clump formation is disclosed in JP-A 8-197597. In JP-A 8-197597, a part such as a kneading disc is provided on the forward end of the metering section in order to decrease the clumps which may remain when the shear force of the screw is insufficient. JP-A 8-197597, however, includes no substantial quantitative description for the length of the fibers remaining in the molded article or the mechanical properties of the molded articles, and the merit achieved by this invention is rather ambiguous. Moreover, this method involves sacrifice of the plasticizing ability, and not all resins can be plasticized by such method, and there is the need for screw-exchange to correspond to the resin type, therefore the method is complicated and undesirable.
As described above, plasticization of the fiber-reinforced thermoplastic resin in a plasticizing apparatus having mounted thereto the plasticizing screw of conventional type had the problem of reduced residual fiber length and insufficient mechanical strength due to the breakage of the reinforcing fibers in the feed section, compression section and metering section while the clump formation could be avoided. On the other hand, the screw wherein shear force had been reduced to prevent the fiber breakage was capable of preventing the fiber breakage. Such screw, however, suffered from the risk of the remaining of the clumps and incapability of resin plasticization due to the reduced plasticization ability.
An object of the present invention is to solve the problems of the prior art as described above, and provide a screw for plasticizing a long fiber-reinforced thermoplastic resin which is capable of preventing both the breakage of the long reinforcing fibers and the clump formation, an apparatus having such screw mounted thereto, a method for molding a long fiber-reinforced thermoplastic resin by using such plasticizing apparatus, and an article molded by such method wherein various mechanical properties have been highly improved by the long reinforcing fibers.